In order to protect a cast-iron or steel plate it is well known to be coated with concrete (see German Pat. No. 471,575, British Pat. Nos. 1,309,525 and 1,504,052, French Pat. No. 2,299,135 and U.S. Pat. No. 1,979,656) as well as to line a metallic pipe with concrete (see British Pat. No. 1,462,888, Japanese Pat. No. 36-23,734, and U.S. Pat. No. 3,087,515). In addition it is known to apply a bitumen to a pipe (see German Pat. Nos. 555,130 and 1,278,899 and U.S. Pat. Nos. 1,948,007 and 1,988,628). It is also known to apply a plastic layer on a concrete pipe (see British Pat. No. 1,244,920) and to provide an epoxy layer on a metallic pipe (see U.S. Pat. No. 3,231,443). U.S. Pat. No. 2,158,771 teaches providing an enamel layer directly on a metal pipe and U.S. Pat. No. 4,058,428 teaches the use of a plastic layer on a thick concrete. Finally, it is known from German Pat. No. 2,743,259 to provide a hydraulic concrete layer on a metallic pipe and to cover this with a moisture-impervious layer.
The reason for thus coating or layering a metallic pipe is to protect it against corrosion. In the last of the above-cited patent documents the combination of a moisture-impervious layer and a concrete layer on a pipe ensures that the basicity of the concrete will protect the pipe, while the moisture-impervious layer will preserve the basicity of the concrete. The problem is, of course, when an uncoated portion of the pipe becomes exposed. In this case the entire covered portion of the pipe acts as a cathode and any uncovered portion as an anode so that when an appropriate electrolyte, such as ground water, is present there will be current flow and corresponding deterioration of the anode surfaces. Such deterioration takes place at a substantially greater rate than normal corrosion.
Consequently, it is extremely important to ensure that the anticorrosion layers applied to the pipe form a continuous hole-free layer. This means not only that the layer must be applied perfectly evenly and continuously, but also that it must stand up under the relatively rough handling such pipe is frequently subjected to.
It has therefore been suggested to apply the hydraulic concrete to the pipe with the assistance of a type saturated with the concrete. Such arrangement holds the concrete securely in place while it cures but does not in fact increase the bonding of the concrete to the pipe. Furthermore even if subsequently covered with a moisture-impervious layer, this layer will inevitably be pierced during shipping and installation of the pipe.